Federal prosecutors are warning Central Valley farmers that a crackdown on large-scale marijuana grow operations is imminent.

The U.S. Attorney’s Office said there has been a significant number of marijuana grows found on agricultural lands in the Central Valley over the last year, especially in Fresno, Kern and Madera counties. U.S. Attorney Benjamin Wagner, from California’s Eastern District, said his office is planning to tackle the grow sites with full enforcement of the federal law prohibiting cultivation and distribution of marijuana.

The crackdown will target those landowners who allow the marijuana grows to operate, Wagner told law enforcement and local agricultural organizations during conferences held Thursday and Friday.

The Department of Justice speculates the proliferation of marijuana grows on agricultural land is in part a result of concentrated efforts by law enforcement to eradicate grow sites hidden on public lands.

During 2011, there were more than 110 identified agricultural grow sites in Fresno County, and another 60 sites were identified in Madera County. These sites ranged in size from 500 to several thousand plants. Kern County reported numerous marijuana agricultural grows that also contained improvised booby traps installed along with the plants. Using modern agricultural techniques, growers living in the fields have produced huge marijuana plants, some reaching a height of 12 feet and weighing several hundred pounds.

Last year a marijuana grow site was discovered in Modesto that yielded 500 marijuana plants weighing more than 3,000 pounds, according to the Department of Justice. An assault rifle also was found at the site.

California’s medical marijuana law removes criminal penalties on the use, possession and cultivation of marijuana at the state level by patients who possess a "written or oral recommendation" from their physician that he or she "would benefit from medical marijuana" with certain provisions.

“These profiteers are not interested in helping sick people. These large commercial operations are profit-driven,” Wagner said. “Much of the marijuana cultivated in the Central Valley is being shipped to other states. We will use our investigative and prosecutorial resources to bring criminal and civil sanctions against those who choose to violate the law.”